Reference for Medical Students, by Medical Students

Our earlier article on ocular ultrasound for measuring intracranial pressure was met with a common question. Primarily, why not just use papilledema as your initial assessment for intracranial pressure? This is what we are taught in medical school. Optic nerve disc swelling equals increased intracranial pressure (ICP). End of Story. Right? Not exactly… While optic disc swelling can indicate increased ICP, it is an inferior measure of acutely elevated ICP. And here is why…

It’s an indirect measure.

It’s a late sign of increased ICP.

It’s more subjective.

It’s not a dynamic measurement.

It’s not always practical.

Our earlier article on ocular ultrasound for measuring intracranial pressure was met with a common question. Primarily, why not just use papilledema as your initial assessment for intracranial pressure? This is what we are taught in medical school. Optic nerve disc swelling equals increased intracranial pressure (ICP). End of Story. Right? Not exactly… While optic disc swelling can indicate increased ICP, it is an inferior measure of acutely elevated ICP. And here is why…

It’s an indirect measure

So just what are you detecting when you see papilledema? It’s not increased CSF pressure causing the optic disc to bulge under pressure. It’s actually the result of optic nerve compression by elevated CSF pressures. The backup of axonal transport caused by compression leads to swelling along the optic nerve and into the optic disc. It is not a direct measure, but a measure of a result of increased intracranial pressure. It is a measure of injury to the optic nerve after increased CSF pressure in the optic nerve sheath has already choked the optic nerve. I don’t know about you, but optic nerve swelling sounds like bad news to me.

On the other hand, ocular ultrasound uses optic nerve sheath diameter to detect increased intracranial pressure. As mentioned in the earlier article (which you should read if you are reading this), the optic nerve sheath is distensible and communicates with the subarachnoid space. By measuring optic nerve sheath diameter you are detecting ICP pressure changes transmitted to the subarachnoid space (and CSF) via the resulting optic nerve sheath distention. This is a much more direct measure of ICP.

It’s a late sign of increased ICP

Ok, so let’s assume for arguments sake you’re alright with using an indirect measure of detection when a more direct measure exists. What now? Let’s examine the time factor. Papilledema takes time to develop. It requires ICP to increase, fluid pressure in the optic nerve sheath to constrict the optic nerve, and axonal transport to back up enough to cause swelling at the optic disc. This can take hours to weeks to manifest. In an emergent situation your acutely ill patient may not have sufficient time to develop papilledema, but still have significantly elevated ICP. If you didn’t understand the pathophysiology of papilledema, you might take this negative exam finding as evidence of normal ICP. Papilledema is a late sign of increased ICP. As we are taught over and over, time is tissue. By the time you see papilledema badness has been going on for a while and tissue is being lost. This is not good!

With ocular ultrasound, you are observing CSF pressure in real time. ICP changes are transmitted to the CSF, including CSF in the optic nerve sheath. Optic nerve sheath diameter (ONSD) reflects distension caused by CSF pressure changes. Since ONSD changes occur concurrently with ICP variations, ONSD reflects the ICP at the time of measurement. This means you see changes in ICP as they develop, and not hours after the fact, using ocular ultrasound.

It’s more subjective

This is another biggie! When I learned physical exam, it took a week of abusing my ever-so-patient boyfriend to see an optic disc clearly on the fundoscopic exam. It took even longer to get used to what a normal optic disc looks like. Seeing the difference between a normal and slightly swollen optic disc through the tiny window of an undilated pupil is vulnerable to subjectivity. A typical emergency room fundoscopic exam doesn’t have a standardized measurement component either. What you think is papilledema someone else may not, so there isn’t much objectivity to it. Even the experts have poor inter-rater agreement on papilledema. When a panel of trained neuro-ophthalmologists were asked to rate the severity of papilledema from images, they only agreed 45% of the time. And this was in images! If the experts of neuro-opthalmology don’t agree on papilledema severity even half of the time, imagine trying to objectively assess a mentally altered patient with undilated pupils as a non-neuro-ophthalmologist. Not fun and definitely not objective.

When I learned to measure ONSD to detect increased ICP, it took less than five minutes. It was fast, user friendly, and objective. Using the caliper function of the ultrasound machine, you can take precise measurements of ONSD. Those measurements have been extensively studied and compared to more invasive ICP monitoring methods. In symptomatic patients, an ONSD >5mm correlates to an ICP >20mmHg. In all patients, an ONSD > 5.7mm correlates to an ICP >20mmHg. No guessing necessary. And as discussed in the prior article, ONSD ultrasound is 100% sensitive and 96% specific for elevated ICP. Now that we know the numbers are objective and validated, what about ONSD measurement variability between users? Measurements can change slightly based the on angle of probe, angle of the patient’s gaze, etc. Despite that, ONSD measurements have good reproducibility between different users (it’s been studied) with a median inter-rater measurement difference of 0.25mm. In an optic nerve sheath at the upper limit of normal (5mm), 0.25 mm variation would only cause a 5% measurement variation. This means inter-rater ONSD measurements on typically overlap by 95%. This is starting to look like a no brainer (pun intended).

It’s not a dynamic measurement

Still not sure..? Papilledema doesn’t provide a dynamic means of monitoring ICP. Well developed cases of papilledema can persist for 6-10 weeks following resolution of elevated ICP. It’s not a great tool for monitoring ICP once papilledema occurs. If papilledema worsens, you can’t distinguish if it’s from worsening ICP or just the natural progression of the papilledema. It is a qualitative means of detecting elevated ICP. Basically it tells you the optic nerve was compressed (possibly by elevated ICP) at some point recently. Another reason not to rely on papilledema as your primary indicator of acute ICP elevation.

As mentioned above, changes in subarachnoid space pressure are reflected in distension of the optic nerve sheath by the CSF within it. This is a dynamic process, happening in real time. As ICP increases or decreases, so do ONSD measurements. This means ONSD reflects the current picture of ICP. For the patient that may develop increased ICP, you can track their ICP status over time with ocular ultrasound. Is ICP improving? Is it worsening? You can monitor this. ONSD measurements provide a quantitative assessment, so you can compare concrete values between ocular scans taken at different times. This makes them more ideal to detect differences in ICP over time.

It’s not always practical

Fundoscopic examination isn’t possible in some patients. A trauma patient with a mechanism consistent with increased ICP may also have ocular damage or periorbital swelling preventing visualization of the optic disc. In these cases papilledema can’t be used to detect increased ICP. Instead ocular ultrasound should be used. Using methods described in our earlier article, ICP can be assessed rapidly and non-invasively in the patient that isn’t even a candidate for fundoscopic exam.

Relevant Anatomy and Physiology

The optic nerve sheath contains the optic nerve and is a potential space that communicates with the subarachnoid space. As ICP pressure increases, the pressure is transmitted to the subarachnoid space and the CSF contained within it. CSF exerting greater pressure within the optic nerve sheath causes it to distend. However its capacity for distension is limited, causing elevated pressure to compress the optic nerve. To create visible papilledema, the pressure within the optic nerve sheath must be great enough to impede axoplasmic transport in the optic nerve. The resulting swelling moves along the nerve and into the optic disc where it is seen on fundoscopic exam. Now I’m just a medical student, but I’m guessing that compressing the optic nerve to the point of swelling is bad. Bad for the nerve. And bad for the patient.

The fundoscopic exam is a great exam and has its place. However papilledema as a method of detecting acutely increased ICP is inferior to ocular ultrasound. Ocular ultrasound provides real time, quantitative measurements. Quantitative measurements make the exam objective and allow you to compare ICP changes over time. It also detects increased ICP earlier than the presence of papilledema, since papilledema takes time to develop. If you do see papilledema during a fundoscopic exam in a patient with suspected increased ICP, remember what a late and serious sign this is. However DO NOT rely on the absence of papilledema to rule out increased ICP. If you think your patient has increased ICP, do a bedside ultrasound. It’s fast, sensitive, objective, provides quantitative information, measures ICP in real time, and is easy to do. Need I say more?